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Indian Remote Sensing Satellite Cartosat-1: Technical features and data products

M.Krishnaswamy* and S. Kalyanaraman**
* Project Director, Cartosat-1, **Programme Director, IRS


Introduction
In the area of Satellite based remote sensing in the past, the first generation satellite IRS-1A and 1B were designed, developed and launched successfully during 1988 and 1991 with multi-spectral cameras with spatial resolution of 72.5 m and 36 m. respectively. Subsequently, the second generation remote sensing satellites IRS-1C and 1D with improved spatial resolutions of 70 m in multi-spectral and 5.8 m. in Panchromatic bands and a wide field sensor with 188m resolution and 800 Km. swath, have been developed and successfully launched in 1995 and 1997 respectively. These satellites have become the principal components in the National Natural Resource Management System and the data was used in various applications, viz., agriculture and soil, land form and land use studies, water resource, forestry, drought and flood monitoring, cartography, town planning and coastal zone monitoring. Especially IRS-1C/D data has been used for cartographic and town planning application up to 1:10,000 scale. These satellites also provide stereo pairs of imageries to get height information to an accuracy of approximately 10 meters.

With the above scenario, India has a lead in the civilian remote sensing field in the world not only in terms of realisation and launching of complex satellites with high, medium and coarse resolution cameras, but also in the application areas as well. In order to maintain this lead and also provide continuity of data to global users, Cartosat-1 with two improved fore and aft PAN cameras with better than 2.5 m. spatial resolution is planned to be realised for launch by middle of 2003. This paper briefly presents the technical elements and the planned data products of the Cartosat-1 spacecraft.

Cartosat-1 Spacecraft Technical Elements:
The spacecraft is configured with the Panchromatic cameras which are mounted such that one camera is looking at +26 deg. w.r.t. nadir and the other at -5 deg. w.r.t. nadir along the track. These two cameras combinedly provide stereoscopic image pairs in the same pass. Also the whole spacecraft is steerable across track to provide wider coverage in a shorter period. A brief description of the payload and the other mainframe elements are given in the subsequent sections.
  1. Remote Sensing Payloads:
    The payload performs the function of imaging an area along the track and transmits the data for ground processing. Each Panchromatic camera consists of three 3 mirror off-axis all reflective telescope with primary, secondary and tertiary mirrors. These mirrors are made from special zerodur glass blanks and are light weighted to about 60%. These mirrors are polished to an accuracy of l/80 and are coated with enhanced AlO2 coating. The mirrors are mounted to the Electro-optical module using iso-static mounts, so that the distortion on the light weighted mirrors are very minimum. The configuration of the electro-optical module of the camera is given in Fig.2.1. In order to meet the high resolution and the swath requirement 12K, 7 micron linear array CCD is planned to be used as a detector. The CCD processing electronics will be using high speed devices to meet the high data rate requirements. Some of the important specifications of the payload are given in Table 2.1.

Fig 2.1 Electro-optical module configuration of pan camera


Table 2.1: Payload Specifications
S.No . Parameter Name SpecificationFore (+26 deg.) Aft (-5 deg)
1. Spatial Resolution:GIFOV (m) (Across-track x along-track) 2.5 x 2.78 2.22 x 2.23
2. Spectral Resolution
a) No. of Bands
b) Bandwidth
1 Panchromatic
500 nm to 850 nm
3. Radiometric Resolution
a) Saturation Radiance
b) Quantisation
c) SNR
55mw/cm*cm/str/micron
10 bits
345 at Saturation Radiance
4. Swath (km) (Stereo)
Fore + Aft Combined (Mono) Km.
30
26.855
5. CCD Parameters:
a) No. of Detectors \ elements
b) Detector Element Size
c) Odd-Even Spacing
12000 per camera
7 x 7 microns
35 microns staggered
6. Optics
a) No. of Mirrors
b) Effective Focal Length (mm)
c) F-Number
d) Field of View (degrees)
3
1980
F/4.5
+/- 1.08
7. Integration Time (ms) 0.336
8. MTF
a) Across track
b) Along track
20
23
9. Onboard Calibration Relative, using LEDs
10 . Data Rate 105 Mb/s
11. Data Compression:
a) Algorithm
b) Compression Ratio
JPEG
Max.3.2
12 Nominal B/H Ratio for Stereo 0.62
14. P/L Operating Temp. Range 20 +/- 1 degree C.

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